1. Field of the Invention
The present invention relates to a method of driving a plasma display panel (hereinafter abbreviated to PDP) or another flat panel display for performing a gradation display in accordance with a period of lighting time.
2. Description of the Prior Art
The PDP obtains a desired display image by controlling light emitting and non-light emitting of each pixel by the use of plasma discharge. The PDP is advantageously thin, and has been prosperously developed as a flat panel display together with a liquid crystal display (abbreviated to LCD) and the like.
In the PDP, a row electrode and a column electrode are formed on a pair of opposed substrates in such a manner that the electrodes intersect each other with a discharge space therebetween on the substrates, and a gap in the substrates is filled with gas for electric discharge. By applying voltage to a discharge cell at an intersecting portion of the column electrode and the row electrode designated in a matrix manner, electric discharge and light emitting are performed. Such spotted electric discharge and light emitting are macroscopically recognized to form a character, a graphic form or another image. In the PDP using such a principle, a light emitting quantity by means of discharge cannot be linearly controlled by the applied voltage. Therefore, multiple gradations are obtained by controlling a lighting time in each pixel in accordance with brightness.
FIG. 1 shows a prior-art constitution of the PDP. Original image data, for example, eight bits data, R, G and B are transmitted to a multi-gradation processor 10, in which an error diffusion process is performed as detailed by applicants of the present application in the U.S. Pat. No. 5,596,349. The data are converted to data of predetermined bits, for example, four bits. The converted original image data are temporarily stored in a frame memory 11, then transmitted to a data controller 12. Predetermined pixel data is then prepared and supplied to a data driver 18 of a PDP display portion 15. On the other hand, a horizontal synchronous pulse SYNC separated from a composite video signal is transmitted to a subfield timing control portion 13, in which signal pulses for controlling column and row, subfield, field, frame and other various timings are prepared and supplied to the multi-gradation processor 10, the frame memory 11, the data controller 12 and a driver controller 14. The driver controller 14 is controlled by the subfield timing control portion 13, to control driving timings of a Y electrode driver 16, an X electrode driver 17 and a data driver 18 of the PDP display portion 15.
In the PDP display portion 15, plural Y electrodes 19 and plural X electrodes 20 are disposed parallel with one another, and intersected by plural data electrodes 21. The Y electrodes 19 and the X electrodes 20 are formed on one substrate, while the data electrodes 21 are formed on the other substrate. The Y and X electrodes 19 and 20 are covered with dielectric layers. On the substrate on which the data electrodes 21 are formed, fluorescent materials R, G and B are provided. In discharge cells constituted of the Y electrodes 19, the X electrodes 20 and the data electrodes 21, respectively, a discharge space is partitioned by barrier ribs constituted of insulation layers formed on the substrates.
The Y electrode driver 16 supplies to the Y electrodes 19 row-directional scanning pulses and common pulses, and the X electrode driver 17 supplies to the X electrodes 20 common pulses. All the X electrodes 20 formed on the substrate are driven in common. Also, the data driver 18 supplies address pulses to the data electrodes 21, thereby addressing the data electrodes 21 in a column direction. The constitution provided with these three types of electrodes is called a three electrode type.
In the three electrode type of PDP, one field is constituted of plural subfields, and each subfield is mainly constituted of an address period and a maintenance discharge period. During each address period, one row constituted of each pair of the Y electrode 19 and the X electrode 20, i.e. one scanning line, is first selected. Specifically, a scanning pulse is applied to the Y electrodes 19 and a sufficiently large voltage is applied between the Y electrode 19 and the X electrode 20. In this condition, a signal voltage is applied from the data driver 18 to a specified data electrode 21, and writing discharge is performed in the discharge cell corresponding to one spot designated in a matrix manner. Thereby, a wall charge is formed on the dielectric layer over the Y electrode 19 and the X electrode 20. Subsequently, during the maintenance discharge period, maintenance discharge pulses are simultaneously applied alternately to the Y electrodes 19 and the X electrodes 20. The wall charge selectively prepared in the address period moves between the Y electrode 19 and the X electrode 20 so as to change its polarity. While maintaining the charge, electric discharge and light emitting are performed. The discharge is repeated bidirectionally in positive and negative directions, thereby lighting to display a sufficient brightness.
In a batch erasing and writing system, at the time of batch writing prior to the address period, the data driver 18 applies signal voltages simultaneously to all the cells and produces wall charges on all the cells. Subsequently, during the address period, the data driver 18 selectively applies erasing pulses. The erasing pulse is smaller in wavelength and amplitude than the maintenance discharge pulse. By applying to the cell to which the erasing pulse has been supplied a voltage with a polarity reverse to that of the wall charge produced at the time of writing, an erasing discharge is performed to erase the wall charge. During the maintenance discharge period, as aforementioned, the maintenance discharge pulses are applied alternately to the Y electrodes 19 and the X electrodes 20, to repeat the maintenance discharge predetermined times.
When the gradation display is performed in the PDP, the brightness of each pixel constituting the discharge cell is controlled by changing a length of the maintenance discharge period which is controlled by a frequency of the maintenance discharge in each discharge cell. Specifically, plural subfields respectively having the maintenance discharge period which is associated with a desired brightness ratio are produced in the subfield timing control portion 13. By allocating each bit of the original image data to these subfields, a combination of subfields to be lit is selected. Specifically, the pixel is only lit in the selected subfield. The total length of the maintenance discharge periods of each pixel is controlled and associated with the gradation of the original image data. The sum of the lighting periods is regarded as a desired display brightness.
A composite video signal for use in output of a TV broadcasting or computer image has heretofore been gamma-compensated in accordance with a display characteristic of a cathode-ray tube (CRT). Therefore, in the case of display on a display unit other than the CRT using the above described composite video signal, brightness is compensated to coincide with a voltage-brightness characteristic inherent in the display unit. Also in the constitution shown in FIG. 1, each of the original image data R, G and B supplied to the multi-gradation processor 10 has a curve of voltage-brightness relationship straightened by applying the gamma-compensation for the PDP further to a signal gamma-compensated at the time of transmitting an image or at the time of output from the computer. In the example, display in sixteen gradations can be obtained from the original image data of four bits. However, the curve of the relationship between the gradation and the display brightness is straight. That is to say, a difference in brightness between the gradations is made equal at all the levels. However, in the case of actual visual observation, human visibility is high in a low brightness region, and the difference in brightness can be clearly recognized. Conversely, in a high brightness region, the visibility is low, and the difference in brightness cannot be easily recognized clearly. Therefore, in the case of visual recognition, the brightness is varied in density across all the brightness regions, and display quality is deteriorated.